Processes, such as vapor deposition of thin metal films on silicon wafers, require the use of high purity gas at high pressures. For example, certain newly developed physical vapor deposition processes utilized by the semiconductor industry require the use of high purity argon at pressures greater than 10,000 psia. Any significant amount of particulate or molecular contaminants, such as various fluorocarbon or hydrocarbon compounds in the argon can contaminate silicon wafer surfaces, and reduce microchip yield to uneconomical levels. Therefore, contamination of argon in such applications must be avoided.
Avoidance of contamination in high pressure argon systems is difficult. A typical means for providing argon at pressures greater than 10,000 psia is through mechanical compression of argon gas. The most reliable mechanical compressors, i.e., those having the longest operating periods between maintenance, use pistons with compression seals to separate the pressurized argon from a hydraulic fluid. Such seals are prone to wear, leak-through, and subsequent contamination of the high purity argon. An alternative compressor design uses an oscillating metal diaphragm to separate the pressurized argon from a hydraulic fluid. However, the diaphragms of such compressors are prone to fatigue failure and require frequent maintenance. Fatigue failure of the diaphragm in such compressors results in contamination of the argon with particles and other impurities.
An alternative means of supplying high pressure argon consists of a two step process in which liquid argon is first compressed to high pressure using a cryogenic liquid pump. The pressurized argon then flows to a separate vessel where heat is transferred into the argon at a fixed, high pressure. The heat transfer raises the temperature of the argon to the ambient level. Using this method, cryogenic liquid pumps can be used to produce argon pressures greater than 10,000 psia as disclosed in U.S. Pat. No. 4,032,337. However, cryogenic liquid pumps require frequent maintenance and liquid sub-cooling to minimize cavitation, and can contaminate the argon with particles or other impurities.
The present invention overcomes the drawbacks of the prior art to avoid contamination of lubricating oils and metals, to avoid the complexity of mechanical compression, and yet provides a simple, clean method of obtaining ultra high pressures in gases having high purity requirements as industry currently demands, as set forth in greater detail below.